Wind is motion of the air over the Earth's surface. It is
driven by the effects of different air densities at different places. These
different densities are caused by an uneven distribution of heating and
cooling over the surface of the Earth which includes lowlands,
mountains, oceans and polar ice caps. The sun can heat a patch of air,
causing it to expand and become less dense. In Earth's gravitational field,
this lighter air tends to rise. Any nearby colder, more dense air moves
horizontally beneath the rising warm air, creating a cool surface wind.

However, the whole process is further complicated by the rotation of the
Earth. The air motion is on such a large scale that circular patterns of air
movements are formed, resulting in rather stable high and low pressure
circular air masses. These rotating air masses tend to drift eastwards at
New Zealand's latitudes, and one often sees these on the TV-weather
reporter's moving, elapsed-time images of daily cloud patterns, as viewed by
weather satellites.

Warm and cold fronts arise from the interactions of these air
masses. As a front passes overhead, there will usually be changing winds at
ground level.

On a smaller scale, localized breezes often occur along the New Zealand
coast. In the daytime, the coastal land is heated by the sun and the air
above it rises. The nearby air over the cool sea remains fairly cold and
dense so it flows in to replace the rising warm air. The wind coming in
from the sea is called the `sea breeze'. Sometimes at night the air over the
land becomes cooler than the air over the sea and this generates a coastal
breeze flowing in the opposite direction, from the land toward the sea.